This invention relates in general to pipe connectors, and particularly to a threaded connector having a separate rigid external seal/support ring affixed to the connector pin for engaging the box.
One method of offshore oil and gas production utilizes a production riser extending upward from a subsea wellhead. The production riser is made up of sections of pipe, typically connected together with threaded connectors. The riser is subject to large tension loads and also to bending moments from currents and waves that the connectors must withstand over long periods of time. The connectors also must seal the internal pressure of the produced fluids and the external pressure of the seawater.
There are two major types of threaded connections used for production risers. In one type, the so-called xe2x80x9cweldedxe2x80x9d type, the pin and box are machined separately from heavy-wall material and then welded to the pipe. In the other type, the so-called xe2x80x9cthreaded-and-coupledxe2x80x9d type, the pin is typically machined directly onto the pipe ends. The box is machined into each end of a coupling that is used to join the pipe ends together.
In both types of connections, a primary internal metal-to-metal seal is formed between a nose of the pin and a mating surface within the box. It is also important to prevent the entry of seawater into the threads using an external seal, preferably a metal-to-metal seal. An external metal-to-metal seal also helps to withstand bending loads.
An external seal is readily accomplished in a welded connection using a metal-to-metal seal located at the opposite end of the threads to prevent the entry of seawater into the threads. The external seal usually requires a tapered surface to be formed on the exterior of the pin and in the interior of the box, thus reducing the metal thickness of the connector in these areas. This connector will have a larger outer diameter than the pipe to accomplish the desired sealing without reducing the ability to withstand stress and fatigue.
In a threaded-and-coupled connection, it is impossible to incorporate an external metal-to-metal seal machined directly onto the pipe without substantially reducing the ability to withstand stress and fatigue. Adhesives and sealants have been used in the past to seal the threads; however, there are disadvantages to this technique. Forging upset pin and box ends to increase the wall thickness available for an external metal-to-metal seal is also known, but is costly.
The threaded-and-coupled riser connector of this invention utilizes a pin that has a separate rigid seal/support ring that is mounted onto the pin to create an external metal-to-metal seal and support for bending loads without substantially reducing the wall thickness of the pipe. The rigid seal/support ring has a seal surface that mates with a seal surface formed in the box. Preferably the rigid seal/support ring is of a different material and is selected to have a lower modulus of elasticity than the pin.
The pin is machined directly on both ends of a pipe. The box is formed in a coupling that secures to the pin. The coupling has two box-threaded sections for connecting between two riser pipes. In the preferred embodiment the rigid seal/support ring is of metal and is installed on the pin end by a shrink- or press-fit. Then, a tapered seal surface is machined on the exterior of the rigid seal/support ring.
The threads are preferably of a buttress type, having a stabbing flank, a load flank and a crest. The root has an elliptically formed junction with the load flank.